If a person is suspected of having a viral infection that is potentially dangerous, the safest action for the public is to quarantine the infected person. Unfortunately, it is difficult to tell whether the infection is from a particularly dangerous strain of a virus by observing symptoms alone. The difficulty when dealing with a potentially dangerous viral outbreak is amplified when those outbreaks are frequent and widespread. The SARS coronavirus spread widely in 2003, and a much larger H1N1 swine flu pandemic followed in 2009. More recently, Ebola has appeared in Africa. MERS has been found in people in the Middle East, and the H5N1 bird flu that has killed many birds has proven able to spread from birds to people.
It may be possible to use DNA sequencing to identify viruses or their specific strains and even to help develop vaccines. However, viruses are capable of rapid mutation and may even benefit from having changing genetic sequences by being better able to evade the host's immune system. For example, new flu vaccines are recommended each year because the influenza virus mutates so rapidly. Moreover, we now understand that a virus may exist in a host as a “quasispecies”—a group of a very large number of closely-related viral genomes that is suited to evade immune destruction by virtue of the inclusion of many random but novel variants any one of which may survive an immune response. Thus when a person is infected with a virus, not only might the virus represent a significant public health threat, the nature of its rapidly mutating genome can make it very difficult to identify and treat.